Two major problems arise with common usage, however. First, they tend to be language-specific, requiring translation into equivalent, or almost-equivalent, terms in other languages. They are not universal terms that may be readily understood by zoologists speaking other languages. Differences in terminology remain a problem that, to some extent, still separates the fields of zoological anatomy (sometimes called zootomy) and human (medical) anatomy.

The second, and larger, problem is caused by the very nature of animals. Most animals are capable of moving relative to their environment (see Fig. 1). So while "up" might refer to the top of someone's head when they are standing upright, the same term ("up") would describe their belly while they are lying down.

Therefore, standardized anatomical (and zootomical) terms of location have been developed, usually based on Latin words, to enable all biological and medical scientists to precisely delineate and communicate information about animal (including human) bodies and their component organs.

Contents

Because animals can change orientation with respect to their environment, and because any appendages (arms, legs, tentacles, etc...) can change position with respect to the main body, it is important that any positional descriptive terms refer to the organism when it is in its standard anatomical position.

Thus, and very importantly, all descriptions are with respect to the organism in its standard anatomical position, even when the organism in question has appendages in another position. For example, see Fig. 8, where the tentacles are curved, and therefore not in anatomical position. However, a straight position is assumed when describing the proximo-distal axis. This helps avoid confusion in terminology when referring to the same organism in different postures.

There is no formal definition of standard anatomical position used in most zoology. However, the position can be loosely defined as that position in which the organism will usually be found when at rest. Thus, for most invertebrates, this would be the position in which they are normally found when not feeding, hiding, actively moving, and so on (see Figs. 6–9, below), and any appendages are straight.

For bilaterally-symmetrical organisms, such vertebrates or some invertebrates, this can be refined to include that the organisms are standing erect in a normal posture, and looking forward.[1] (For example, see Figs. 2–4, below.)

Unlike the situation in zootomy, standard anatomical position is rigidly defined for human anatomy. As with other vertebrates, the human body is standing erect and at rest. Unlike the situation in other vertebrates, the limbs (arms and legs) are placed in unnatural positions reminiscent of the supine position imposed on cadavers during autopsy. Therefore, the body has its feet together (or slightly separated), and its arms are rotated outward so that the palms are forward, and the thumbs are pointed away from the body (forearms supine). As well, the arms are usually moved slightly out from the body, so that the hands do not touch the sides.[2][3] The positions of the limbs (and the arms in particular) have important implications for directional terms in those appendages.

In humans, the anatomical position of the skull has been agreed by international convention to be the Frankfurt plane, a position where the lower margins of the orbits and the upper margins of the ear canals all lie in the same horizontal plane. This is a good approximation to the position where the subject is standing upright and facing forwards.

Ultimately, the bodies we are most familiar with are vertebrate bodies similar to our own. All vertebrates (including humans) have the same basic body plan (or bauplan) — they are bilaterally symmetrical. That is, they have mirror-image left and right halves if divided down the centre.[4][5][6][7] For these reasons, the basic directional terms can be considered to be those used in vertebrates. By extension, the same terms are used for many other (invertebrate) organisms as well.

To begin, distinct, polar-opposite ends of the organism are chosen. By definition, each pair of opposite points defines an axis. In a bilaterally-symmetrical organism, there are 6 polar opposite points, giving three axes that intersect at right angles — the x, y, and z axes familiar from three-dimensional geometry.

The most obvious end-points are the "nose" and "tail" (see Fig. 2). Anatomically, the nose is referred to as the anterior end (Latinante; before). In organisms like vertebrates, that have distinct heads, the anterior end is sometimes referred to as the rostral end (Latin rostrum; beak), the cranial end (Greek kranion; skull), or the cephalic end (Greekkephalē; head).[8][9][10] For reasons of broader applicability, especially in organisms without distinct heads (many invertebrates), "anterior" is usually preferred.[11][12][13]

The polar opposite to the anterior end is the posterior end (Latinpost; after). Another term for posterior is caudal (Latin caudum; tail) — a term which strictly applies only to vertebrates, and therefore less preferred.[14][15][16]

By drawing a line connecting these two points, we define the anteroposterior axis (sometimes written antero-posterior). Less-used synonyms would be rostrocaudal or cephalocaudal axes (see Table 1). For brevity, the term anteroposterior is often abbreviated to read AP (or A-P) axis. As well as defining the anteroposterior axis, the terms "anterior" and "posterior" also define relative positions along the axis. Thus, in the fish in Fig. 2, the gill openings are posterior relative to the eyes, but anterior to the tail.

The next most obvious end-points are the back and belly. These are termed the dorsal end (Latin dorsum; back) and the ventral end (Latin venter; abdomen), respectively. By connecting the outermost points the dorsoventral axis is formed (sometimes hyphenated: dorso-ventral). This is commonly abbreviated to DV (or D-V) axis. The DV axis, by definition, is perpendicular (at right angles to) the AP axis at all times (see below).

As with anteroposterior, the terms "dorsal" and "ventral" are also used to describe relative positions along the dorsoventral axis. Thus, the pectoral fins are dorsal to the anal fin, but ventral to the dorsal fin in Fig. 2. (Note that these fins are not aligned anteroposteriorly, either — the dorsal fin being posterior to the pectoral, and anterior to the anal fins, respectively.)

The last axis, by geometric definition, must be at right angles to both the AP and DV axes. Obviously, the left side and right side of the organism are the outermost points between the two "sides" of the organism. When connected, these points form the left-right axis (commonly abbreviated to LR (or L-R) axis. Properly, this is called the dextro-sinistral (or, more uncommonly, the sinistro-dextral) axis, from the Latindexter (right) and sinister (left). It is important to note that the "left" and "right" sides are the sides of the organism, and not those of the observer.

In practice, and contradictory to the practice with other anatomical terms of location, the vernacular "left-right" is preferentially used in English and some other languages. This is likely due to the adoption of the Latin "sinister" to mean "evil" in English[17] and other languages (e.g. sinistre in French has the same connotation[18]).

As with the other directions, the terms can be used as relative terms, to describe locations along the left-right axis. Thus, in Fig. 2 the dorsal fin is right of the left pectoral fin, but is left of the right eye. However, as left and right sides are mirror images, usage like this tends to be somewhat confusing, as structures are duplicated on both sides (i.e. above there is both a right eye and a left eye, forcing one to specify which is used as a reference).

To counter this clumsiness of usage, the directional term lateral (Latinlateralis; "to the side") is used as a modifier for both sides, yielding the left lateral and right lateral sides. As an opposite to lateral, the term medial (Latinmedius; "middle") is used to define a point in the centre of the organism (where the left-right axis intersects the midsagittal plane — see below). Thus, rather than "left-right" axis and its inherent clumsiness of usage, the term mediolateral (also sometimes hyphenated medio-lateral) axis is frequently used. Sometimes this is abbreviated to ML (or M-L) axis.[19][20][21] Properly, the ML axis is a half axis; practically, its usage is less clumsy and less linguistically biased than "left-right". The terms may still be used relatively to describe locations along the LR axis. Thus, in Fig. 2 the gills are medial to the operculum, but lateral to the heart.

The usage "mediolateral" is strictly used to describe relative position along the left-right axis, to avoid confusion with the terms "superficial" and "deep" (see below).

Together, the AP, DV and LR (or ML) axes allow for precise three-dimensional descriptions of location within any bilaterally-symmetrical organism, whether vertebrate or invertebrate. In practice, the terms can cause some confusion when, unlike the fish shown in Fig. 2, the organism in question is not strictly linear in form (see Figs. 3 and 4). For example, the AP axis in Fig. 3 does not appear to be at right angles to the DV axis. Rather, it is a depiction of the approximate average AP axis, when all body segments are included.

When considering any one segment, the dorsoventral axis is perpendicular to the AP axis. Thus, in Fig. 4, the DV axis of the tail would run from the "back" of the tail (posterior end of the trunk), to the "underside" of the tail (near the legs) — nearly parallel to the AP axis of the main body.

As a general rule of thumb, if the body is included in consideration, the AP axis of the main body would be used, as would the DV and ML axes perpendicular to it. However, if considering only one segment, the AP axis would shift to reflect the axes shown in Fig. 4, with the DV and ML axes shifting correspondingly. Alternatively, to avoid confusion, AP, DV and ML terms are used strictly in relation to the main body, and the terms proximal and distal are used for body segments such as the head, neck and tail (see below).

The term proximal (Latinproximus; nearest) is used to describe where the appendage joins the body, and the term distal (Latin distare; to stand away from) is used for the point furthest from the point of attachment to the body. Since appendages often move independently of (and therefore change position with respect to) the main body, these separate directional terms are used when describing them.

As noted above, the standard AP, DV and ML directional axes, can cause some confusion when describing parts of the body that can change position (move) relative to the main body. This is particularly true when considering appendages. "Appendages" would include vertebratefins (see Fig. 2) and limbs (see Figs. 3 and 4), but properly apply to any structure that extends (and can at least potentially move separately) from the main body. Thus, "appendage" would also include such structures as external ears (pinneae) and hair (in mammals), feathers (in birds) and scales (fish, reptiles and birds). As well, varieties of tentacles or and other projections from the body in invertebrates and the male penis in many vertebrates and some invertebrates, would be included.

By connecting the two points, the proximodistal (sometimes hyphenated to proximo-distal) axis. (The abbreviation AB axis is occasionally, but not commonly, used.) As before, the terms "proximal" and "distal" can be used as relative terms to indicate where structures lie along the proximodistal axis. Thus, the "elbow" is proximal to the hoof, but distal to the "shoulder" in Figs. 3 and 4.

Choosing terms for the other two axes perpendicular to the proximodistal axis could be variable, as they would also depend on the position of the limb. For that reason, when considering any organism, the other two axes are considered to be relative to the appendage when in standard anatomical position. This is roughly defined for all organisms, as in the normal position when at rest and not moving. For tetrapodvertebrates, this includes the caveat that they are standing erect and not lying down. Thus, the fish in Fig. 2, and the horse in Figs. 3 and 4 are in standard anatomical position. (Special considerations with respect to limb position are applied in human anatomy — see below).

In addition to the three primary axes (AP, DV and the ML half-axis) and the proximodistal axis of appendages, several directional terms can be used in bilaterally symmetrical animals. These terms are strictly relative, and as such do not and cannot be used to define fixed axes. These terms include:

Ipsilateral (Latinipse; self/same): on the same side as another structure. Thus, the left arm is ipsilateral to the left leg.

Contralateral (Latin contra; against): on the opposite from another structure. Thus, the left arm is contralateral to the right arm, or the right leg.

Superficial (Latin superfacies; at the surface or face): near the outer surface of the organism. Thus, skin is superficial to the muscle layer. The opposite is "deep", or "visceral".

Deep: further away from the surface of the organism. Thus, the muscular layer is deep to the skin, but superficial to the intestines. This is one of the few terms where the English vernacular is prevalent. The proper anglicised Latin term would be profound (Latin profundus; due to depth), but this word has other meanings in English. In other languages, the equivalent term is usually similar to "profound" (e.g. profond, meaning deep, in French).

Intermediate (Latin intermedius; inter, between and medius, middle): between two other structures. Thus, the navel is intermediate to (or intermediate between) the left arm and the contralateral (right) leg.

Visceral (Latin viscus; internal organs, flesh): organs within the body's cavities. The stomach is within the abdominal cavity, and is thus visceral.

The large variety of body shapes present in invertebrates presents a difficult problem when attempting to apply standard directional terms. Depending on the organism, some terms are taken by analogy from the vertebrate terms, and appropriate novel terms are applied, as necessary. In all cases, the usage of terms is dependent on the bauplan of the organism.

In organisms with a changeable shape, such as amoeboid organisms (Fig. 5a), directional terms are meaningless, since the shape of the organism is changeable, and no fixed axes are present. Similarly, in organisms that are spherical in shape (Fig. 5b), there is nothing to distinguish one line through the centre of the organism from another. An infinite number of triads of mutually perpendicular axes could be defined, but any such choice of axes would be functionally and practically indistinguishable from all others, and therefore would be useless. In such organisms, only the terms superficial and deep hold any descriptive meaning.

In organisms that maintain a constant shape and have one dimension longer than the other, at least two directional terms can be used. The long or longitudinal axis is defined by points at the opposite ends of the organism. Similarly, a perpendicular transverse axis can be defined by points on opposite sides of the organism. There is typically no basis for the definition of a third axis. Usually such organisms, like that pictured in Fig. 6, are planktonic (free-swimming) protists, and are nearly always viewed on microscope slides, where they appear essentially two-dimensional. In some cases a third axis can be defined, particularly where a non-terminal cytostome or other unique structure is present.[22]

Some elongated protists have distinctive ends of the body. In such organisms, the end with a mouth (or equivalent structure, such as the cytostome in Paramecium or Stentor), or the end that usually points in the direction of the organism's locomotion (such as the end opposite the flagellum in Euglena), is normally designated as the anterior end. The opposite end then becomes the posterior end, and by connecting them, an anteroposterior axis is formed.[23] Properly, this terminology would only apply to an organism that is always planktonic (not normally attached to a surface — as in Fig. 6 top), although the term can also be applied to one that is sessile (normally attached to a surface — as in Fig. 6, bottom and Fig. 7).[24]

Organisms that are attached to a substrate, such as sponges (Fig. 7), or some animal-like protists also have distinctive ends. The part of the organism attached to the substrate is usually referred to as the basal end (Latinbasis; support or foundation), whereas the end furthest from the attachment is referred to as the apical end (Latin apex; peak, tip). Thus, by joining the two ends, an apical-basal (or basal-apical) axis is formed (see Fig. 7). Transverse axes may be defined indifferently in any direction perpendicular to this axis, as there is no symmetry present.

Cnidarians have an incomplete digestive system, meaning that one end of the organism has a mouth, and the opposite end has no opening from the gut (coelenteron).[29] For this reason, the end of the organism with the mouth is referred to as the oral end (Latinoris; mouth), and the opposite surface is the aboral end (Latin ab-; prefix meaning "away from"). Thus, by joining the polar opposite oral and aboral ends, an oral-aboral axis is formed (Fig. 8).

As with vertebrates, appendages that move independently of the body (tentacles in cnidarians and comb jellies), have a definite proximodistal axis (Fig. 8). Unlike vertebrates, cnidarians (jellyfish, sea anemones, corals) have no other distinctive axes, and multiple radial axes are possible (Fig. 9).

It is noteworthy that some "biradially-symmetrical" comb jellies have distinct "tentacular" and "pharyngeal" axes,[30] and are thus anatomically equivalent to bilaterally-symmetrical animals. As well, adult echinoderms (starfish, sea urchins, sea cucumbers) are pentaradial, and have only five symmetrical radial axes (unlike the multiple axes in cnidarians).

"Lateral", dorsal, and ventral have no meaning in such organisms, and all can be replaced by the generic term peripheral (Latin peri-; around; see Table 2). Medial can be used, but in the case of radiates indicates the central point of these organisms, rather than a central axis (as in vertebrates). Thus, as there are many possible radial axes, there are multiple medio-peripheral (half-) axes (Fig. 9).

Notes:(1) Includes both Radiates and adult Echinoderms.(2) Rarely used.(3) Only in organisms attached to a substrate.(4) Vertebrate equivalents are meaningless in radial animals.(5) Roughly equivalent to "superficial". (6) Roughly equivalent to "deep".

As we are bilaterally-symmetrical organisms, anatomical directions in humans can correctly be described using the same terms as those for vertebrates and other members of the taxonomic group Bilateria. However, for historical and other reasons, standard human directional terminology has several differences from that used for other bilaterally-symmetrical organisms.

Although it can be argued that the standard directional nomenclature used for vertebrate zootomy can and should be used for medical anatomy, the differences persist. The differences in terminology arose (and are perpetuated) for three primary reasons:

Early human anatomical studies (being within the realm of medicine) were historically conducted separately from, and without reference to, those being done by zootomists.

Early zoological and human anatomical studies occurred before modern understanding of the process of biological evolution, and humans were widely viewed as "different" from (and "superior to") all other animals, and thus meriting their own terminology.

Unlike most tetrapod vertebrates, humans are not quadrupedal (walking on four legs), but rather are secondarily bipedal (walking on two legs). Human bipedalism causes shifts in the angle of the appendages (arms and legs) and head, with respect to the main body. Thus, it can be (and is) argued that separate terminology is necessary to adequately describe the unique bipedal stance of humans.

Unfortunately, the persistence of medical terminology as distinct from that used for other vertebrates tends to be confusing. For a quick comparison of equivalent terminology used in vertebrate and human anatomy, see Table 3 (below).

As with other vertebrates, two of the most obvious extremes are the "top" and the "bottom" of the organism. In standard anatomical position, these correspond to the head and feet, respectively in humans. The head end is referred to as the superior end (Latinsuperior: "above"), while the feet are referred to as the inferior end (Latin inferior: "below"). Thus, the axis formed by joining the two is the superior-inferior axis.[31][32]

As in other vertebrates, there are synonymous terms for superior and inferior (Table 3). The terms cranial, cephalic, and rostral are occasionally encountered. "Cranial", as a reference to the skull, is fairly commonly used, whereas "cephalic" is uncommonly used, and "rostral" is rarely used in human anatomy.[33] Similarly, the term caudal is occasionally used in human anatomy,[34] and the cranio-caudal axis is occasionally encountered. Generally, this usage would only be used with respect to the head and main body (trunk), and not when considering the limbs.

As with vertebrate directional terms, superior and inferior can be used in a relative sense. For example, the shoulders are superior to the navel, but inferior to the eyes.

Anterior and posterior, as used in medical/human anatomical descriptions are major sources of confusion to those accustomed to standard vertebrate directional terminology, and vice versa. The confusion arises from the differences in standard anatomical positions of quadruped vertebrates and bipedal humans.

In human anatomical usage, anterior refers to the "front" of the individual, and is synonymous with ventral. Similarly, posterior, in medical anatomy refers to the "back" of the subject, and is synonymous with dorsal (see Table 3).[35] The terms "dorsal" and "ventral" are used in human anatomy, but infrequently when referring to the body as a whole.[36] Thus, the anteroposterior axis is preferred usage for describing the axis connecting the front and the back in humans.[37][38]

As in other vertebrates, "anterior" and "posterior" can also be used as relative terms. Thus, the eyes are posterior to the nose, but anterior to the back of the head.

Left and right lateral are used in the same sense as they are in other vertebrates, as is medial. The left-right axis is rarely used in medicine, however; the mediolateral axis is used almost exclusively.[39][40]

As in other vertebrates, the terms "proximal" and "distal" are used to describe the point of attachment to, and part of an appendage furthest away from, the body, respectively. However, other terms are used for direction in the appendages, given the unique position of the limbs (in standard anatomical position) in humans.

In standard anatomical position, the palms of the hands point anteriorly. Thus, anterior can be (and sometimes is) used to describe the palm of the hand, and posterior can be (and sometimes is) used to describe the back of the hand and arm.

However, presumably for improved clarity, the directional term palmar (Latinpalma; palm of the hand) is usually used for the anterior of the hand, and dorsal is used to describe the back of the hand. Thus, by connecting the extremes, dorsopalmar axis is formed. Most commonly, "dorsopalmar" is used when describing the hand, although it is sometimes applied to the arm as a whole (see Fig. 11).

For the third axis, the mediolateral axis suffices, although if referring to the limb alone, "medial" may refer to the centre of the arm itself.

Specialized terms are used to describe location on appendages, parts that have a point of attachment to the main trunk of the body. Structures that are close to the point of attachment of the body are proximal or central, while ones more distant from the attachment point are distal or peripheral. For example, the hands are at the distal end of the arms, while the shoulders are at the proximal ends. These terms can also be used relatively to organs, for example the proximal end of the urethra is attached to the bladder.

In the limbs of most animals, the terms cranial and caudal are used in the regions proximal to the carpus (the wrist, in the forelimb) and the tarsus (the ankle in the hindlimb). Objects and surfaces closer to or facing towards the head are cranial; those facing away or further from the head are caudal.

Nearer the carpal joint, the term dorsal replaces cranial and palmar replaces caudal. Similarly, nearer the tarsal joint the term dorsal replaces cranial and plantar replaces caudal. For example, the top of a dog's paw is its dorsal surface; the underside, either the palmar (on the forelimb) or the plantar (on the hindlimb) surface.

The sides of the forearm are named after its bones: Structures closer to the radius are radial, structures closer to the ulna are ulnar, and structures relating to both bones are referred to as radioulnar. Similarly, in the lower leg, structures near the tibia (shinbone) are tibial and structures near the fibula are fibular (or peroneal).

Volar (sometimes used as a synonym for "palmar") refers to the underside, for both the palm and the sole (plantar), as in volar pads on the underside of hands, fingers, feet and toes.

The terms valgus and varus are used to refer to angulation of the distal part of a limb at a joint. For example, at the elbow joint, in the anatomical position, the forearm and the upper arm do not lie in a straight line, but the forearm is angulated laterally with respect to the upper arm by about 5–10°. The forearm is said to be "in valgus". Angulation at a joint may be normal (as in the elbow) or abnormal.

A sagittal plane divides the body into sinister and dexter (left and right) portions.

The midsagittal or median plane is in the midline — i.e. it would pass through midline structures such as the navel or spine, and all other sagittal planes (also referred to as parasagittal planes) are parallel to it.

A coronal or frontal plane divides the body into dorsal and ventral (back and front, or posterior and anterior) portions.

A transverse plane, also known as an axial plane or cross-section, divides the body into cranial and caudal (head and tail) portions.

For post-embryo humans a coronal plane is vertical and a transverse plane is horizontal, but for embryos and quadrupeds a coronal plane is horizontal and a transverse plane is vertical.

When describing anatomical motion, these planes describe the axis along which an action is performed. So by moving through the transverse plane, movement travels from head to toe. For example, if a person jumped directly up and then down, their body would be moving in the transverse plane.

Some of these terms come from Latin. Sagittal means "like an arrow", a reference to the position of the spine which naturally divides the body into right and left equal halves, the exact meaning of the term "midsagittal".

A longitudinal plane is any plane perpendicular to the transverse plane. The coronal plane and the sagittal plane are examples of longitudinal planes.

A transverse (also known as axial or horizontal) plane is an X-Y plane, parallel to the ground, which (in humans) separates the superior from the inferior, or put another way, the head from the feet.

A coronal (also known as frontal) plane is an Y-Z plane, perpendicular to the ground, which (in humans) separates the anterior from the posterior, the front from the back, the ventral from the dorsal.

A sagittal (also known as median) plane is an X-Z plane, perpendicular to the ground, which separates left from right. The midsagittal plane is the specific sagittal plane that is exactly in the middle of the body.

The axes and the sagittal plane are the same for bipeds and quadrupeds, but the orientation of the coronal and transverse planes switch. The axes on particular pieces of equipment may or may not correspond to axes of the body, especially since the body and the equipment may be in different relative orientations.

Occasionally, in medicine, abdominal organs may be described with reference to the trans-pyloric plane which is a transverse plane passing through the pylorus.

In discussing the neuroanatomy of animals, particularly rodents used in neuroscience research, a simplistic convention has been to name the sections of the brain according to the homologous human sections. Hence, what is technically a transverse (orthogonal) section with respect to the body length axis of a rat (dividing anterior from posterior) may often be referred to in rat neuroanatomical coordinates as a coronal section, and likewise a coronal section with respect to the body (i.e. dividing ventral from dorsal) in a rat brain is referred to as transverse. This preserves the comparison with the human brain, whose length axis in rough approximation is rotated with respect to the body axis by 90 degrees in the ventral direction. It implies that the planes of the brain are not necessarily the same as those of the body.
Actually, the situation is more complex, since comparative embryology shows that the length axis of the neural tube (the primordium of the brain) has three internal bending points, namely two ventral bendings at the cervical and cephalic flexures (cervical flexure roughly between the medulla oblongata and the spinal cord, and cephalic flexure between the diencephalon and the midbrain), and a dorsal (pontine or rhombic) flexure at the midst of the hindbrain, behind the cerebellum. The latter flexure mainly appears in mammals and sauropsids (reptiles and birds), whereas the other two, and principally the cephalic flexure, appear in all vertebrates (the sum of the cervical and cephalic ventral flexures is the cause of the 90 degree angle mentioned above in humans between body axis and brain axis. This more realistic concept of the longitudinal structure of vertebrate brains implies that any section plane, except the sagittal plane, will intersect variably different parts of the same brain as the section series proceeds across it (relativity of actual sections with regard to topological morphological status in the ideal unbent neural tube). Any precise description of a brain section plane therefore has to make reference to the anteroposterior part of the brain to which the description refers (e.g., transverse to the midbrain, or horizontal to the diencephalon). A necessary note of caution is that modern embryologic orthodoxy indicates that the brain's true length axis finishes rostrally somewhere in the hypothalamus where basal and alar zones interconnect from left to right across the median line; therefore, the axis does not enter the telencephalic area, although various authors, both recent and classic, have assumed a telencephalic end of the axis. The causal argument for this lies in the end of the axial mesoderm -mainly the notochord, but also the prechordal plate- under the hypothalamus. Early inductive effects of the axial mesoderm upon the overlying neural ectoderm is the mechanism that establishes the length dimension upon the brain primordium, jointly with establishing what is ventral in the brain (close to the axial mesoderm) in contrast with what is dorsal (distant from the axial mesoderm). Apart of the lack of a causal argument for introducing the axis in the telencephalon, there is the obvious difficulty that there is a pair of telencephalic vesicles, so that a bifid axis is actually implied in these outdated versions.

In humans, reference may be made to landmarks which are on the skin or visible underneath. As with planes, lines and points are imaginary. Examples include:

The mid-axillary line, a line running vertically down the surface of the body passing through the apex of the axilla (armpit). Parallel are the anterior axillary line, which passes through the anterior axillary skinfold, and the posterior axillary line, which passes through the posterior axillary skinfold.

Tuffier's line, which is a transverse line passing across the lumbar spine between the posterior iliac crests.

Mid-ventral line, the intersection between the ventral skin and the median plane.

Additionally, reference may be made to structures at specific levels of the spine (e.g. the 4th cervical vertebra, abbreviated "C4"), or the rib cage (e.g. the 5th intercostal space, abbreviated "5ICS").